SAW filter and electronic device including SAW filter

ABSTRACT

A surface acoustic wave (SAW) filter includes an input terminal, an output terminal, a series resonator provided between the input terminal and the output terminal, first and second parallel resonators having respective one ports connected to respective ports of the series resonator, respectively, first and second nodes connected to respective other ports of the first and second parallel resonators, respectively, first and second inductance elements having respective one ends connected to the first and second nodes, respectively, a third node connected to respective other ends of the first and second inductance elements, a third inductance element having one end connected to the first node, a grounding terminal connected to other end of the third inductance element, and a capacitance element connected between the first and second nodes. The SAW filter has a large attenuation amount at a lower side of a pass band of the filter, thus having sufficient properties.

FIELD OF THE INVENTION

[0001] The present invention relates to a SAW filter and an electronicdevice including the SAW filter for use in a communications apparatus.

BACKGROUND OF THE INVENTION

[0002] In a conventional ladder-type surface acoustic wave (SAW) filter,as shown in FIG. 10, series resonators 3, 4, and 5 are connected inseries between an input terminal 1 and an output terminal 2, andparallel resonators 6 and 7 are connected to respective ports of theseries resonator 4. The parallel resonators 6 and 7 are connected toinductance elements 8 and 9, respectively. The inductance elements 8 and9 are connected to one end of an inductance element 10. The other end ofthe inductance element 10 is connected to a grounding terminal 11.

[0003] A characteristic profile of the conventional ladder-type SAWfilter having the above-described arrangement is shown by a line B inFIG. 4. As shown as the line B, the conventional SAW filter has a widepass band and has a attenuation profile of an amplitude at both sides ofthe pass band.

[0004] The attenuation profile of the conventional SAW filter is notsufficient particularly at a lower side of the pass band. That is, asshown as the line B in FIG. 4, an attenuation amount tends to declineand then increase at the lower side of the pass band.

SUMMARY OF THE INVENTION

[0005] A surface acoustic wave (SAW) filter includes an input terminal,an output terminal, a series resonator provided between the inputterminal and the output terminal, first and second parallel resonatorshaving respective one ports connected to respective ports of the seriesresonator, respectively, first and second nodes connected to respectiveother ports of the first and second parallel resonators, respectively,first and second inductance elements having respective one endsconnected to the first and second nodes, respectively, a third nodeconnected to respective other ends of the first and second inductanceelements, a third inductance element having one end connected to thefirst node, a grounding terminal connected to other end of the thirdinductance element, and a capacitance element connected between thefirst and second nodes.

[0006] The SAW filter has a large attenuation amount at a lower side ofa pass band of the filter, thus having sufficient properties.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a circuit diagram of a SAW filter according to anexemplary embodiment of the present invention.

[0008]FIG. 2 is a plan view of the SAW filter of the embodiment.

[0009]FIG. 3 is a cross sectional view of the SAW filter of theembodiment.

[0010]FIG. 4 is a frequency characteristic of the SAW filter of theembodiment.

[0011]FIG. 5 is a circuit diagram electrically equivalent to that of theSAW filter of the embodiment.

[0012]FIG. 6 is a plan view of another SAW filter of the embodiment.

[0013]FIG. 7 is a plan view of still another SAW filter of theembodiment.

[0014]FIG. 8 is a plan view of a further SAW filter of the embodiment.

[0015]FIG. 9 is a block diagram of an electronic device including theSAW filter of the embodiment.

[0016]FIG. 10 is a circuit diagram of a conventional SAW filter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0017]FIG. 1 illustrates a circuit diagram of a surface acoustic wave(SAW) filter according to an exemplary embodiment of the presentinvention. Series resonators 14, 15, and 16 are connected in series inthis order between an input terminal 12 and an output terminal 13.Parallel resonators 17 and 18 have respective one ports connected torespective ports of the series resonator 15. The parallel resonators 17and 18 have respective other ports connected to a first grounding node19 and a second grounding node 20. A first inductance element 22 isconnected between the first grounding node 19 and the second groundingnode 20 while a second inductance element 23 is connected between thesecond grounding node 20 to a third grounding node 21. A thirdinductance element 25 is connected between the third grounding node 21and a grounding terminal 24. A capacitance element 26 is connectedbetween the first grounding node 19 and the second grounding node 20.

[0018] A characteristic profile of the SAW filter is shown as a line Ain FIG. 4. The SAW filter has a pass band ranging from 2.11 to 2.17 GHzwith little loss and has a large attenuation at both sides of the passband. As shown, the attenuation in the SAW filter is greaterparticularly at the lower side of the pass band than that in aconventional SAW filter denoted by the line B.

[0019] A reason will now be explained referring to FIGS. 3 and 5.

[0020]FIG. 3 is a cross sectional view of the SAW filter of theembodiment. The first inductance element 22 and the second inductanceelement 23 are formed of wires. The third inductance element 25 isprovided as an electrode pattern formed on a package 50.

[0021]FIG. 5 illustrates a circuit diagram equivalent to the circuit ofthe SAW filter of the embodiment.

[0022] The attenuation at the lower side of the pass band in the SAWfilter depends a lot on the inductance elements 22, 23, and 25. Thus,the attenuation of the conventional SAW filter is adjusted by adjustingthe inductances of the inductance elements. As shown in FIG. 3, theinductances are substantially defined by the shape of the package.Meanwhile, the conventional SAW filter has an attenuation firstdeclining and then sprung up at the lower side of the pass band as shownin FIG. 4.

[0023] The inductance elements 22, 23, and 25 shown in FIG. 1 arearranged in a Y-shaped circuit in which the first grounding node 19, thesecond grounding node 20, and the grounding terminal 24 are coupled viathe third grounding node 21 as a center of the circuit. The Y-shapedcircuit is equivalently converted into a fourth inductance element 27connected between the first grounding node 19 and the second groundingnode 20, a fifth inductance element 28 connected between the firstgrounding node 19 and a grounding terminal 30, and a sixth inductanceelement 29 connected between the second grounding node 20 and agrounding terminal 31.

[0024] An inductance L₁ of the first inductance element 22, aninductance L₂ of the second inductance element 23, an inductance L₃ thethird inductance element 25, an inductance L₄ of the fourth inductanceelement 27, an inductance L₅ of the fifth inductance element 28, and aninductance L₆ of the sixth inductance element 29 satisfy the followingrelation:

L ₄=(L ₁ L ₂ +L ₂ L ₃ +L ₃ L ₁)/L ₃;

L ₅=(L ₁ L ₂ +L ₂ L ₃ +L ₃ L ₁)/L ₂; and

L ₆=(L ₁ L ₂ +L ₂ L ₃ +L ₃ L ₁)/L ₁.  (Equation 1)

[0025] The inductance L₃ is generally much smaller than the inductancesL₁ and L₂. Accordingly, the inductance L₅ is substantially equal to theinductance L₁ based on Equation 1. Similarly, the inductance L₆ issubstantially equal to the inductance L₂. The inductance L₄ issignificantly large.

[0026] The circuit shown in FIG. 5 includes the inductance element 27connected in parallel with the capacitance element 26. This arrangementallows an frequency characteristic of an impedance of the inductanceelement 27 to be adjusted by the capacitance of the capacitance element26. As the result, the attenuation at the lower side of the pass bandcan be controlled without modifying of the series resonators 14, 15, and16 and the parallel resonators 17 and 18.

[0027] Thus, the attenuation at the lower side of the pass band canexplicitly be large as denoted by the line A in FIG. 4.

[0028] Since the inductance L₄ is significantly large, the capacitanceof the capacitance element 26 maybe significantly small. This allows thecapacitance element 26 to be fabricated by a method similarly to thatfor fabricating the series resonators 14, 15, and 16 and the parallelresonators 17 and 18. The method will be explained below.

[0029]FIG. 2 illustrates the series resonators 14, 15, and 16 and theparallel resonators 17 and 18 in the circuit shown in FIG. 1. Theresonators include electrodes formed on a piezoelectric substrate 51. Asshown in FIG. 2, the capacitance element 26 is implemented by twointer-digital electrodes 26 extending toward each other from the firstand second grounding nodes 19 and 20, respectively, on the piezoelectricsubstrate 51.

[0030]FIGS. 6, 7, and 8 illustrate other electrodes of the capacitanceelement 26. The two electrodes of the capacitance element 26 extendtowards each other from the grounding nodes 19 and 20, respectively. Theelectrodes may be electrodes 26 a having respective long sides simplyprojecting as shown in FIG. 6. The electrodes may be L-shaped electrodes26 b extending towards each other to have respective long sides facingeach other, as shown in FIG. 7. The electrodes may be electrodes 26 cfacing each other, as shown in FIG. 8.

[0031] The SAW filter according to the embodiment includes three of theseries resonators 14 to 16, however may not includes at least one of theseries resonators 14 and 16.

[0032]FIG. 10 illustrates an electronic device including the SAW filterof the embodiment and other components. FIG. 10 is a block diagram of aradio-frequency unit of a movable communication apparatus. A SAWduplexer 53 includes SAW filters 53A and 53B having respective pass banddifferent from each other. The radio-frequency unit includes an antenna52, the SAW duplexer 53 (the SAW filters 53A and 53B), a coupler 54 forconnected to input port 153A, 153B, and output port 253A, 253B of theSAW filters 53A, 53B, and electronic components, such as a poweramplifier 55 and a low noise amplifier (LNA) 56. The SAW filter 53Bconnected to the LNA 56 generally has the pass band higher than that ofthe SAW filter 53A. A signal amplified with an power amplifier 55 isradiated through the antenna 52 efficiently since each SAW filter has alarge attenuation at the lower side of the pass band, thus providing theelectronic device including the SAW filters 53A and 53B of theembodiment with excellent characteristics.

What is claimed is:
 1. A surface acoustic wave (SAW) filter comprising:an input terminal; an output terminal; a series resonator providedbetween said input terminal and said output terminal; first and secondparallel resonators having respective one ports connected to respectiveports of said series resonator, respectively; first and second nodesconnected to respective other ports of said first and second parallelresonators, respectively; first and second inductance elements havingrespective one ends connected to said first and second nodes,respectively; a third node connected to respective other ends of saidfirst and second inductance elements; a third inductance element havingone end connected to said first node; a grounding terminal connected toother end of said third inductance element; and a capacitance elementconnected between said first and second nodes.
 2. The SAW filteraccording to claim 1, further comprising: a piezoelectric substrate forforming at least one of said series resonator, said first parallelresonator, and said second parallel resonators.
 3. The SAW filteraccording to claim 2, further comprising: electrodes provided on saidpiezoelectric substrate and facing each other for forming saidcapacitance element.
 4. The SAW filter according to claim 2, whereinsaid first and second nodes are formed on said piezoelectric substrateand facing each other for forming said capacitance element.
 5. The SAWfilter according to claim 2, further comprising: projecting electrodesprovided on said piezoelectric substrate and extending towards eachother from said first and second nodes, respectively, for forming saidcapacitance element.
 6. The SAW filter according to claim 5, whereinsaid projecting electrodes includes inter-digital electrodes.
 7. Anelectronic device comprising: a surface acoustic wave (SAW) filterincluding an input terminal, an output terminal, a series resonatorprovided between said input terminal and said output terminal, first andsecond parallel resonators having respective one ports connected torespective ports of said series resonator, respectively. first andsecond nodes connected to respective other ports of said first andsecond parallel resonators, respectively, first and second inductanceelements having respective one ends connected to said first and secondnodes, respectively, a third node connected to respective other ends ofsaid first and second inductance elements, a third inductance elementhaving one end connected to said first node, a grounding terminalconnected to other end of said third inductance element, and acapacitance element connected between said first and second nodes; andan electronic component connected to one of said input terminal and saidoutput terminal.
 8. The electronic device according to claim 7, whereinsaid SAW filter further includes a piezoelectric substrate for formingat least one of said series resonator, said first parallel resonator,and said second parallel resonators.
 9. The electronic device accordingto claim 8, wherein said SAW filter further includes electrodes providedon said piezoelectric substrate and facing each other for forming saidcapacitance element.
 10. The electronic device according to claim 8,wherein said first and second nodes are formed on said piezoelectricsubstrate and facing each other for forming said capacitance element.11. The electronic device according to claim 8, wherein said SAW filterfurther includes projecting electrodes provided on said piezoelectricsubstrate and extending towards each other from said first and secondnodes, respectively, for forming said capacitance element.
 12. The. SAWfilter according to claim 11, wherein said projecting electrodesincludes inter-digital electrodes.